Image recording apparatus and method of generating pixel clock

ABSTRACT

An image is accurately recorded by a light beam on a PS plate wound on a drum which is rotating at a constant speed. A rotary encoder detects information of a recording position in a main scanning direction by the light beam that is emitted from an optical unit to the PS plate. Based on the detected information, a PLL circuit of a recording synchronizing signal generating unit generates an original clock. Pulses of the original clock are counted by a decimating counter, which outputs a decimating instruction to decimate a pulse from the original clock each time the count reaches a preset count. Based on the decimating instruction, a pulse is decimated from the original clock, and a decimated clock is frequency-divided at a fixed frequency-dividing ratio by a frequency divider, which outputs a pixel clock for recording the image. Since the frequency of the pixel clock is varied by decimating the original clock based on the preset count, the image can accurately be recorded on the PS plate by determining in advance the preset count depending on the positional relationship between the PS plate and the optical unit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image recording apparatushaving an image recording means for recording a two-dimensional image ona recording sheet such as a photosensitive medium held on the outer orinner circumferential surface of a drum, an a method of generating apixel clock, which is preferably applicable to such an image recordingapparatus.

[0003] 2. Description of the Related Art

[0004] There have heretofore been known external surface scanning lightbeam image recording apparatus for recording a two-dimensional image onthe entire surface of a recording medium on the outer circumferentialsurface of a cylindrical drum by rotating the drum, scanning therecording medium with an intensity-modulated light beam emitted from anoptical system in a main scanning direction, and moving the opticalsystem in an axial direction of the drum thereby to scan the recordingmedium in an auxiliary scanning direction transverse to the mainscanning direction. See, for example, Japanese laid-open patentpublications Nos. 5-207250, 9-149211, and 10-16290, for details.

[0005] The drum, which has a diameter of 300 mm and a length of 1 m andis made of aluminum or the like, of those disclosed external surfacescanning light beam image recording apparatus actually suffers variousdimensional errors. For example, the drum has various diameter and outercircumferential surface dimension variations, which fall within amachining tolerance range, caused in the manufacturing process, and alsohave eccentricity errors introduced when the drums are assembled.Consequently, even when the drum is rotated at a constant speed, thecircumferential speed of the outer circumferential surface of the drumis not constant. With the irregular circumferential speed, when an imageis plotted on the photosensitive medium by the light beam that isintensity-modulated, e.g., selectively turned on and off, with pixelclock pulses at constant intervals, the recorded image tends to beunduly expanded or contracted in local regions.

[0006] One solution proposed in the known apparatus has been to measurea distortion of an image which has been plotted with pixel clock pulsesand correcting the spaced intervals of the pixel clock pulses when theimage is actually recorded for thereby minimizing expansions andcontractions of the image. According to the system disclosed in Japaneselaid-open patent publication No. 5-207250, the frequency-diving ratio ofa PLL circuit which generates pixel clock pulses is varied to correctthe spaced intervals of the pixel clock pulses. However, the disclosedsolution is disadvantageous in that the image tends to be distorted dueto a pull-in time of the PLL circuit at the time the frequency-divingratio thereof is varied.

[0007] The technique revealed in Japanese laid-open patent publicationNo. 9-149211 corrects the spaced intervals of pixel clock pulses bychanging an input voltage applied to a voltage-controlled oscillator.The revealed technique is also problematic in that the image is liableto suffer a new distortion owing to the temperature characteristics ofthe voltage-controlled oscillator.

[0008] It has been proposed to use a programmable delay line or aplurality of delay lines to correct clock pulse positions for solvingthe problem disclosed in Japanese patent laid-open publication No.5-207250 or Japanese laid-open patent publication No. 9-149211. However,a correcting circuit made of inexpensive delay line or lines fails toachieve a required level of accuracy and resolution.

[0009] To eliminate the above difficulties, the system disclosed inJapanese laid-open patent publication No. 10-16290 employs, as shown inFIGS. 13 and 14 of the accompanying drawings, a rotary encoder 1 mountedon the shaft of a motor for rotating the drum to generate a fundamentalclock whose frequency is multiplied to produce an original clock by aPLL circuit 2. The pulses of the original clock are digitally counted bya counter 3. The counter 3 comprises a preset down counter and functionsas a frequency divider, and is also referred to as a frequency divider.Based on the count from the counter 3, a CPU 4 reads correcting datafrom a correcting data memory 5. Based on the read correcting data, acontrol circuit 6 selects a frequency-dividing ratio of the counter orfrequency divider 3 to divide the frequency of the original clock fromthe PLL circuit 2 by 7, 8, or 9.

[0010] The disclosed system can achieve a required level of accuracy andresolution because the clock pulse positions are corrected digitally bythe counter 3 and a clock adjusting means 7 which is made up of the CPU4, the corrective data memory 5, and the control circuit 6.

[0011] The correcting data are produced as follows: The circumferentialsurface of the drum that corresponds to a full image surface isdeveloped into a flat rectangular surface, which is divided along mainand auxiliary scanning directions into a mesh pattern of smallrectangular cells or grip points, and correcting data for the respectiverectangular cells or grid points are stored as original correcting datain the correcting data memory 5. The CPU 4 calculates, from coordinatesto be recorded next that are obtained by counting pixel clock pulses andthe stored original correcting data, correcting data for the coordinateposition to be recorded next, and determines a recording time based onthe calculated correcting data.

[0012] However, the above technique is disadvantageous in that when anexposure recording condition such as a dot per inch (DPI) with respectto the photosensitive medium is changed, it is necessary to calculateand regenerate original correcting data for respective grid points ofthe full image surface, and hence the productivity is greatly reduced.

[0013] The foregoing drawback may be eliminated by generating originalcorrecting data for respective grid points of the full image surfacewith respect to each exposure recording condition and storing thegenerated original correcting data in the correcting data memory. Thisapproach is highly costly because a large-storage-capacity semiconductormemory or a hard disk is needed as the correcting data memory forstoring such original correcting data.

[0014] If clock pulse positions are to be corrected in view of theexpansion or contraction of the drum due to environmental temperaturechanges, then it is necessary to store original correcting data for eachtemperature, resulting in a possible further increase in the cost. Thesystem shown in FIGS. 13 and 14 is also problematic in that it requiresa complex control process for the control circuit 6 to setfrequency-dividing ratios in the counter or frequency divider 3 forsmall variations of clock pulse positions to be corrected, the CPU 4requires a considerable power to generate a correcting table for settingfrequency-dividing ratios, and the correcting data memory 5 needs alarge storage capacity for storing the calculated data.

[0015] In addition, the original clock outputted from the PLL circuit 2,whose frequency is 8 times the frequency of the pixel clock, is usuallyfrequency-divided by 8 and partly frequency-divided by 7 or 9 by thecounter or frequency divider 3, for the correction of pixel clockpositions. Therefore, pixel clock positions are corrected in fixedpositions along the main scanning direction at all times, so that animage produced on the photosensitive medium tends to suffer a qualitydegradation such as a striped irregularity or a moiré pattern.

SUMMARY OF THE INVENTION

[0016] It is therefore an object of the present invention to provide animage recording apparatus which has a relatively simple arrangementcapable of stably correcting a distortion of an image, such as anexpansion or a contraction, due to an error of a mechanical system forholding a recording sheet for recording the image thereon, for therebyaccurately recording or reproducing the image on the recording sheet,and a method of generating a pixel clock in such an image recordingapparatus.

[0017] Another object of the present invention is to provide an imagerecording apparatus which is capable of generating and holding data forcorrecting the level of graphical accuracy efficiently with a resourcesaver, against a distortion of an image, such as an expansion or acontraction, due to an error of a mechanical system for holding arecording sheet for recording an image thereon, and a method ofgenerating a pixel clock in such an image recording apparatus.

[0018] Still another object of the present invention is to provide animage recording apparatus which will not produce a quality degradationsuch as a striped irregularity or a moiré pattern in a recorded image,and a method of generating a pixel clock in such an image recordingapparatus.

[0019] According to the present invention, there is provided an imagerecording apparatus has image recording means for scanning a recordingsheet in a main scanning direction to record an image on the recordingsheet, the image recording means being movable in an auxiliary scanningdirection Y substantially perpendicular to the main scanning direction Xto record a two-dimensional image on the recording sheet. The imagerecording apparatus comprises means for detecting recording positioninformation in the main scanning direction, original clock generatingmeans for generating an original clock based on the recording positioninformation in the main scanning direction, decimation counting meansfor counting pulses of the original clock and outputting a decimatinginstruction to decimate a pulse from the original clock each time apreset count is reached, decimating means for decimating a pulse fromthe original clock based on the decimating instruction, andfrequency-dividing means for frequency-dividing a decimated clock at afixed frequency-dividing ratio and outputting the frequency-dividedclock as a pixel clock for recording the image.

[0020] With the above arrangement, since the frequency of the pixelclock is varied by decimating the original clock based on the presetcount, the image can accurately be recorded on the recording sheet bydetermining in advance the preset count depending on the positionalrelationship between the recording sheet and the image recording means.

[0021] The recording sheet may comprise a photosensitive medium such asPS plate, a photosensitive film, or the like, or a printing sheet ofpaper, or a metal plate such as an aluminum sheet or the like.

[0022] The image recording means may comprise an optical system foremitting a light beam to be applied to the recording sheet. The opticalsystem allows pixels having a diameter of 10 μm or less to be producedwith the light beam emitted thereby. If a PS plate is used as therecording sheet, it allows the image recording apparatus to beconstructed as a CTP (Computer To Plate) apparatus.

[0023] The image recording means may comprise an ink jet head 134 forapplying an ink I to the recording sheet, and the image recordingapparatus may further comprise a rotatable drum with the recording sheetmounted on an outer circumferential surface thereof, means forcontrolling the ink jet head to apply the ink to scan the recordingsheet on the rotatable drum in the main scanning direction to record theimage on the recording sheet, and means for moving the ink jet head inthe auxiliary scanning direction Y along an axis of the rotatable drumto record the two-dimensional image on the recording sheet. With thisarrangement, the image recorded on the recording sheet can maintain adesired level of dimensional accuracy irrespective of variations of thediameter of the drum.

[0024] The image recording apparatus may further comprise a rotatabledrum with the recording sheet mounted on an outer circumferentialsurface thereof, means for controlling the optical system to apply thelight beam to scan the recording sheet on the rotatable drum in the mainscanning direction to record the image on the recording sheet, and meansfor moving the optical system in the auxiliary scanning direction alongan axis of the rotatable drum to record the two-dimensional image on therecording sheet. With this arrangement, the image recorded on therecording sheet can maintain a desired level of dimensional accuracyirrespective of variations of the diameter of the drum.

[0025] The image recording apparatus may further comprise a drum withthe recording sheet mounted on an inner circumferential surface thereof,means for rotating the optical system about an axis of the drum to causethe light beam emitted from the optical system to scan the recordingsheet on the rotatable drum in the main scanning direction to record theimage on the recording sheet, and means for moving the optical system inthe auxiliary scanning direction along the axis of the drum to recordthe two-dimensional image on the recording sheet. With this arrangement,the image recorded on the recording sheet can maintain a desired levelof dimensional accuracy irrespective of variations of the diameter ofthe drum.

[0026] The image recording apparatus may further comprise means fordetecting information per revolution of the drum, the decimationcounting means comprising means for resetting the count of the originalclock and thereafter starting to count the original clock to the presetcount when the information per revolution of the drum is detected.Therefore, if necessary, a correcting value can be varied for each mainscanning line thereby to facilitate a fine correcting process.

[0027] Similarly, the image recording apparatus may further comprisemeans for detecting information per revolution of the optical system,the decimation counting means comprising means for resetting the countof the original clock and thereafter starting to count the originalclock to the preset count when the information per revolution of thedrum is detected. Therefore, if necessary, a correcting value can bevaried for each main scanning line thereby to facilitate a finecorrecting process.

[0028] The image recording apparatus may further comprise random numbergenerating means for generating a random number, the decimation countingmeans comprising means for setting a first preset count of the originalclock after the count is reset to a value corresponding to the randomnumber generated by the random number generating means, and outputting adecimating instruction to set a second and subsequent preset count ofthe original count to the preset count. Consequently, pixel clockpositions are prevented from being corrected in fixed positions alongthe main scanning direction at all times, so that an image produced onthe recording sheet does not suffer a quality degradation such as astriped irregularity or a moiré pattern.

[0029] The first preset count of the original clock after the count isreset may be set to the random number between a value of 0 and thepreset value. The random number generating means is thus simple instructure, and corrected positions are prevented from being displacedlargely.

[0030] The preset count may be determined depending on either one of adiameter of the drum, a temperature of the image recording apparatus, ora thickness of the recording sheet. Thus, images to be recorded canaccurately be corrected with respect to such various parameters.

[0031] According to the present invention, there is also provided amethod of generating a pixel clock to correct a graphical accuracydistortion of an image recorded on a recording sheet in an imagerecording apparatus which has image recording means for scanning arecording sheet mounted on a mechanical component in a main scanningdirection to record an image on the recording sheet per pulse of thepixel clock, the image recording means being movable in an auxiliaryscanning direction substantially perpendicular to the main scanningdirection to record a two-dimensional image on the recording sheet, themethod comprising the steps of generating mechanical componentcorrecting data based on a relative positional relationship between themechanical component and the image recording means, and controlling atime to generate the pixel clock to energize the image recording meansbased on the mechanical component correcting data and proportionalcomponent correcting data corresponding to recording conditions for theimage recording means to record the image on the recording sheet, whenthe image is recorded on the recording sheet by the image recordingmeans.

[0032] With the above arrangement, the time to generate the pixel clockis controlled based on the mechanical component correcting data based onthe relative positional relationship between the mechanical componentand the image recording means and the proportional component correctingdata Dp corresponding to recording conditions for the image recordingmeans to record the image on the recording sheet.

[0033] The mechanical component correcting data include main andauxiliary scanning direction components each kept for one line, and theproportional component correcting data is not kept but recalculated eachtime recording conditions are determined. Therefore, the amount ofcorrecting data that is generated and held is minimized.

[0034] The proportional component correcting data comprises either oneof data of the recording resolution on the recording sheet, thethickness of the recording sheet, and the temperature in the imagerecording apparatus, for example.

[0035] The image recording means may comprise an optical system foremitting a light beam to be applied to the recording sheet.

[0036] The image recording means may comprise an ink jet head forapplying an ink to the recording sheet, the image recording apparatusfurther comprising a rotatable drum with the recording sheet mounted onan outer circumferential surface thereof, means for controlling the inkjet head to apply the ink to scan the recording sheet on the rotatabledrum in the main scanning direction to record the image on the recordingsheet, and means for moving the ink jet head in the auxiliary scanningdirection along an axis of the rotatable drum to record thetwo-dimensional image on the recording sheet. With this arrangement, theimage recorded on the recording sheet can maintain a desired level ofdimensional accuracy irrespective of variations of the diameter of thedrum.

[0037] The mechanical component may comprise a rotatable drum with therecording sheet mounted on an outer circumferential surface thereof, theimage recording apparatus further comprising means for controlling theoptical system to apply the light beam to scan the recording sheet onthe rotatable drum in the main scanning direction to record the image onthe recording sheet, and means for moving the optical system in theauxiliary scanning direction along an axis of the rotatable drum torecord the two-dimensional image on the recording sheet.

[0038] The mechanical component may comprise a rotatable drum with therecording sheet mounted on an inner circumferential surface thereof, theimage recording apparatus further comprising means for rotating theoptical system about an axis of the drum to cause the light beam emittedfrom the optical system to scan the recording sheet on the rotatabledrum in the main scanning direction to record the image on the recordingsheet, and means for moving the optical system in the auxiliary scanningdirection along the axis of the drum to record the two-dimensional imageon the recording sheet.

[0039] The mechanical component correcting data may preferably begenerated as main scanning component correcting data for correcting agraphical accuracy distortion in a circumferential direction of thedrum, and auxiliary scanning component correcting data for correcting agraphical accuracy distortion in an axial direction of the drum.

[0040] According to the present invention, there is further provided animage recording apparatus comprising image recording means for scanninga recording sheet mounted on a mechanical component in a main scanningdirection to record an image on the recording sheet per pulse of a pixelclock, the image recording means being movable in an auxiliary scanningdirection substantially perpendicular to the main scanning direction torecord a two-dimensional image on the recording sheet, means fordetecting recording position information in the main scanning direction,original clock generating means for generating an original clock basedon the recording position information in the main scanning direction,decimation counting means for counting pulses of the original clock andoutputting a decimating instruction to decimate a pulse from theoriginal clock each time a preset count is reached, decimating means fordecimating a pulse from the original clock based on the decimatinginstruction, frequency-dividing means for frequency-dividing a decimatedclock at a fixed frequency-dividing ratio and outputting thefrequency-divided clock as a pixel clock for recording the image,storage means for storing mechanical component correcting data based ona relative positional relationship between the mechanical component andthe image recording means, and decimating value calculating means forcalculating the preset count from the mechanical component correctingdata stored in the storage means and proportional component correctingdata corresponding to recording conditions for the image recording meansto record the image on the recording sheet, and setting the calculatedpreset count in the decimation counting means.

[0041] Since the decimating value calculating means calculates thepreset count set in the decimation counting means from the mechanicalcomponent correcting data stored in the storage means and proportionalcomponent correcting data corresponding to recording conditions for theimage recording means to record the image on the recording sheet, datafor correcting the graphical accuracy can be generated and heldefficiently with a resource saver against a distortion of an image, suchas an expansion or a contraction, due to an error of a mechanical systemfor holding the recording sheet.

[0042] The image recording apparatus may further comprise means fordetecting information per revolution of the drum, and random numbergenerating means for generating a random number. The decimation countingmeans may comprise means for, when the count of the original clock isreset and thereafter the original clock starts to be counted to thepreset count when the information per revolution of the drum isdetected, setting a first preset count of the original clock after thecount is reset to a value corresponding to the random number generatedby the random number generating means, and outputting a decimatinginstruction to set a second and subsequent preset count of the originalcount to the preset count. Consequently, pixel clock positions areprevented from being corrected in fixed positions along the mainscanning direction at all times, so that an image produced on therecording sheet does not suffer a quality degradation such as a stripedirregularity or a moiré pattern.

[0043] The above and other objects, features, and advantages of thepresent invention will become more apparent from the followingdescription when taken in conjunction with the accompanying drawings inwhich preferred embodiments of the present invention are shown by way ofillustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044]FIG. 1 is a perspective view, partly in block form, a light beamimage recording apparatus according to an embodiment of the presentinvention;

[0045]FIG. 2 is a block diagram of a recording synchronizing signalgenerating unit in the light beam image recording apparatus shown inFIG. 1;

[0046]FIG. 3 is a perspective view of an internal scanning type lightbeam image recording apparatus according to another embodiment of thepresent invention;

[0047]FIG. 4 is a block diagram of another recording synchronizingsignal generating unit for use in the light beam image recordingapparatus;

[0048]FIG. 5 is a flowchart of an operation sequence of the recordingsynchronizing signal generating unit shown in FIG. 4;

[0049]FIG. 6 is a front elevational view of a test chart;

[0050]FIG. 7 is a simplified block diagram of the recordingsynchronizing signal generating units;

[0051]FIG. 8 is a perspective view, partly in block form, an externalscanning type ink jet image recording apparatus according to stillanother embodiment of the present invention;

[0052]FIG. 9 is a block diagram of still another recording synchronizingsignal generating unit for use in the light beam image recordingapparatus;

[0053]FIG. 10 is a flowchart of an operation sequence of the recordingsynchronizing signal generating unit shown in FIG. 9;

[0054]FIG. 11 is a block diagram of yet another recording synchronizingsignal generating unit for use in the light beam image recordingapparatus;

[0055]FIG. 12 is a block diagram of yet still another recordingsynchronizing signal generating unit for use in the light beam imagerecording apparatus;

[0056]FIG. 13 is a block diagram of a conventional system; and

[0057]FIG. 14 is a detailed block diagram of the conventional systemshown in FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058]FIG. 1 schematically shows a light beam image recording apparatus10 according to an embodiment of the present invention.

[0059] As shown in FIG. 1, the light beam image recording apparatus 10has a drum 14. A presensitized (PS) plate or recording sheet 12 as aphotosensitive medium for recording an image thereon is wounded on theouter circumferential surface 16 of the drum 14. The PS plate 12 iswound around the outer circumferential surface 16 of the drum 14 andheld intimately against the outer circumferential surface 16 by a holder(not shown).

[0060] In this embodiment, the drum 14 comprises a cylindrical drumhaving a diameter of 300 mm and a length of 1 m and made of aluminum.

[0061] The drum 14 has an axial shaft 18 having one end connected to amain scanning motor 20 which comprises an AC servomotor as a drivesource for rotating the drum 14 about its own axis at a constant speedin the main scanning direction indicated by the arrow X. The mainscanning motor 20 has a shaft 22 extending remotely from and coaxiallywith the shaft 18 and coupled to a rotary encoder 24 which rotates inunison with the drum 14 and functions as a means for detecting arecording position in the main scanning direction X.

[0062] The rotary encoder 24 outputs A-phase pulses Pa and Z-phasepulses Pz representing detected recording position information in themain scanning direction X. The rotary encoder 24 outputs 5000 A-phasepulses Pa per revolution of the drum 14 and one Z-phase pulse Pz perrevolution of the drum 14. Therefore, the rotary encoder 24 whichoutputs Z-phase pulses Pz also functions as a means for detectinginformation (origin information) per revolution of the drum 14.

[0063] A-phase pulses Pa and Z-phase pulses Pz outputted from the rotaryencoder 24 are supplied to a mechanical control unit 26 as a means forcontrolling rotary drive sources, and also to a recording synchronizingsignal generating unit 30 of an exposure control unit 28. The recordingsynchronizing signal generating unit 30 functions as a means forgenerating a recording synchronizing signal (pixel clock), and theexposure control unit 28 functions as an exposure control means. Each ofthe mechanical control unit 26 and the recording synchronizing signalgenerating unit 30 is implemented by a microcomputer including a CPUserving as a control means.

[0064] The light beam image recording apparatus 10 also has a ball screw32 extending parallel to the shaft 18 and an optical unit 34 mounted onthe ball screw 32 and a guide rail (not shown) parallel to the ballscrew 32. The optical unit 34 provides an optical system functioning asan image recording means. The optical unit 34 comprises a laser diode 36as a laser beam emitting means for generating a light beam L comprisinga light beam, and a focusing optical system 38 for focusing the lightbeam L emitted from the laser diode 36 onto the PS plate 12 on the drum14.

[0065] An auxiliary scanning motor 40 such as a stepping motor or thelike is connected to an end of the ball screw 32 as a rotary drivesource for rotating the ball screw 32 about its own axis to translatethe optical unit 34 along the shaft 18 of the drum 14 in an auxiliaryscanning direction indicated by the arrow Y.

[0066] An origin detector 42 for detecting an origin or home position ofthe optical unit 34 in the auxiliary scanning direction Y is fixedlypositioned near the end of the drum 14 from which the shaft 18 projects.The origin detector 42 supplies a detected-origin signal Sz indicativeof the detected origin or home position of the optical unit 34 in theauxiliary scanning direction Y to the mechanical control unit 26.

[0067] Based on an instruction signal from a host computer, thedetected-origin signal Sz from the origin detector 42, A-phase pulsesPa, and Z-phase pulses Pz, the mechanical control unit 26 rotates themain scanning motor 20 at a constant speed. Each time the mechanicalcontrol unit 26 is supplied with a Z-phase pulse Pz, the mechanicalcontrol unit 26 rotates the auxiliary scanning motor 40 in one step tofeed the optical unit 34 stepwise in the auxiliary scanning direction Y.

[0068] Basically, the exposure control unit 28 has the recordingsynchronizing signal generating unit 30 and a memory 44 functioning asan image data storage means which uses, as a reading signal, a pixelclock CKi that is a recording synchronizing signal supplied from therecording synchronizing signal generating unit 30.

[0069] The memory 44 stores image data Di, which comprises halftone dotimage data to be recorded on the PS plate 12, supplied from the hostcomputer.

[0070] The memory 44 outputs image data Di, which is binary gradationdata of “0” or “1”, read using the pixel clock CKi supplied from therecording synchronizing signal generating unit 30 as a reading address.The outputted image data Di is supplied to a laser diode drive circuitor LD driver 46 which functions as a drive means or optical system drivemeans for energizing the laser diode 36. The LD driver 46 supplies anon/off signal corresponding to the image data Di (the on/off signal isoff when the image data Di is Di=0 and on when the image data Di isDi=1) to the laser diode 36.

[0071] The laser diode 36 emits a light beam L which is turned on or offdepending on the supplied on/off signal. The light beam L is thenapplied via the focusing optical system 38 to the PS plate 12 therebyrecording an image, i.e., a halftone dot image, on the PS plate 12 basedon the image data Di while the PS plate 12 is being rotated in the mainscanning direction X.

[0072] The light beam image recording apparatus 10 is basicallyconstructed as described above.

[0073] General operation of the light beam image recording apparatus 10will be described below.

[0074] In the light beam image recording apparatus 10 shown in FIG. 1,when the optical unit 34 is positioned at the origin in the auxiliaryscanning direction Y, the mechanical control unit 26 rotates the mainscanning motor 20 at a constant speed to rotate the drum 14 and hencethe PS plate 12 mounted on the outer circumferential surface 16 of thedrum 14. The constant-speed rotation of the main scanning motor 20 isachieved by the mechanical control unit 26 according to a feedbackcontrol process based on A-phase pulses Pa from the rotary encoder 24.

[0075] While the drum 14 is being rotated at a constant speed, theauxiliary scanning motor 40 is turned a predetermined angular intervaleach time a Z-phase pulse Pz is applied, thereby feeding the opticalunit 34 stepwise in the auxiliary scanning direction Y. When the opticalunit 34 is fed to its origin in the auxiliary scanning direction Y, theorigin detector 42 generates and supplies a detected-origin signal Sz inthe auxiliary scanning direction Y to the mechanical control unit 26.

[0076] The mechanical control unit 26 in turn supplies thedetected-origin signal Sz to the recording synchronizing signalgenerating unit 30.

[0077] The recording synchronizing signal generating unit 30 supplies apixel clock CKi generated from A-phase pulses Pa to the memory 44. Basedon the pixel clock CKi, stored image data Di is read from the memory 44and supplied to the LD driver 46, which energizes the optical unit 34.The optical unit 34 applies a light beam L which is selectively turnedon and off as a recording beam to the PS plate 12.

[0078] With the light beam image recording apparatus 10 shown in FIG. 1,the light beam L emitted from the optical unit 34 and applied to the PSplate 12 mounted on the outer circumferential surface 16 of the drum 14that is rotated at a constant speed by the main scanning motor 20 isselectively turned on and off while scanning the PS plate 12 in the mainscanning direction X to record a linear image on the PS plate 12 alongeach main scanning line. At the same time, the optical unit 34 is movedin the auxiliary scanning direction Y by the auxiliary scanning motor 40to record a two-dimensional image, i.e., halftone dot image, on the PSplate 12.

[0079] The light beam image recording apparatus 10 generally operates inthe manner described above.

[0080]FIG. 2 shows in block form the recording synchronizing signalgenerating unit 30 functioning as a means for generating a pixel clock.As shown in FIG. 2, the recording synchronizing signal generating unit30 has a phase-locked loop (PLL) circuit 50. The PLL circuit 50 includesa series-connected circuit which comprises a phase comparator 50 ahaving an input terminal that is supplied with A-phase pulses Pa fromthe rotary encoder 24, a low-pass filter (LPF) 50 b, and avoltage-controlled oscillator (VCO) 50 c, and a frequency divider 50 dfor frequency-dividing an output from the voltage-controlled oscillator50 c and supplying a frequency-divided signal to the other inputterminal of the phase comparator 50 a. The PLL circuit 50 functions asan original clock generating means for generating an original clock CKathat comprises multiplied pulses (multiplied by 10) in synchronism withthe A-phase pulses Pa.

[0081] The original clock CKa is supplied to a count input terminal of adecimating counter 52, which functions as a decimation counting meansfor counting pulses of the original clock CKa and outputting adecimating instruction Sa to decimate a pulse from the original clockCKa each time a preset count Sb is reached. The original clock CKa isalso supplied to an input terminal 54 b of a gate circuit 54, whichfunctions as a decimating means for decimating a pulse from the originalclock CKa and outputting a clock CKb from which the pulse has beendecimated according to the decimating instruction Sa.

[0082] The clock CKb is supplied from an output terminal 54 a of thegate circuit 54 to a frequency divider 56, which functions as afrequency-dividing means whose frequency-dividing ratio is fixed to “8”,for example. The frequency divider 56 frequency-divides the clock CKb by8, and outputs a pixel clock CKi for recording an image, i.e., outputsone pulse of the pixel clock CKi each time 8 pulses of the clock CKb aresupplied to the frequency divider 56.

[0083] The decimating counter 52 comprises a preset down counter, andhas a reset input terminal supplied with a Z-phase pulse Pz each timethe drum 14 makes one revolution.

[0084] Z-phase pulses Pz are also supplied to a control terminal 58 d ofa switch 58. The switch 58 comprises one-circuit, two-contact switchhaving control terminals 58 d, 58 e, a common terminal 58 a, and fixedterminals 58 b, 58 c.

[0085] When a Z-phase pulse Pz is supplied to the decimating counter 52,the decimating counter 52 resets, i.e., clears, the count of theoriginal clock Cka, and starts counting pulses of the original clock Ckaagain. When a Z-phase pulse Pz is supplied to the control terminal 58 dof the switch 58, the common terminal 58 a of the switch 58 istemporarily connected to the fixed terminal 58 c, allowing a first countSd in each main scanning line from an initial decimating value register60, which functions as an initial count setting means, to be supplied asa predetermined count Sb (Sb=Sd at this time) to a preset input terminal52 a of the decimating counter 52.

[0086] When pulses of the original clock CKa are counted to the firstcount Sd by the decimating counter 52, the decimating counter 52 outputsa counting end signal as the decimating instruction Sa to a controlterminal 54 d of the gate circuit 54, shifting the output terminal 54 ato an unconnected terminal 54 c for a period of time in which one pulseof the original clock CKa is decimated.

[0087] The counting end signal as the decimating instruction Sa is alsosupplied to the control terminal 58 e of the switch 58, shifting thecommon terminal 58 a from the fixed terminal 58 c to the fixed terminal58 b to allow a second and subsequent count Sc, e.g., Sc=72, which hasbeen set in a decimating value register 62 by a CPU 64, e.g., a one-chipCPU such as a microcomputer, to be set as the preset count Sb in theinput terminal 52 a of the decimating counter 52 (Sb=Sc).

[0088] A random number Se which is equal to or smaller than the secondand subsequent count Sc is set as a first count Sd in each main scanningline in the initial decimating value register 60.

[0089] If a random number Se generated by a random number generator 66that is initiated by a Z-phase pulse Pz or a disagreement output Sf froma comparator 68 is equal to or smaller than the count Sc set in thedecimating value register 62 (Se≦Sc), then the random number Se is setas the first count Sd in the initial decimating value register 60. Ifthe random number Se is in excess of the count Sc set in the decimatingvalue register 62, then a random number Se is generated again by therandom number generator 66 based on the disagreement output Sf from thecomparator 68 until the random number Se becomes equal to or smallerthan the count Sc.

[0090] The recording synchronizing signal generating unit 30 shown inFIG. 2 may be considered as having a clock adjusting means 200 foradjusting the pixel clock CKi which comprises a decimation countingmeans 202 and an initial decimation random number generating means 204.The decimation counting means 202 comprises the decimating counter 52,the switch 58, the decimating value register 62, and the CPU 64, and theinitial decimation random number generating means 204 comprises theinitial decimating value register 60, the random number generator 66,and the comparator 68.

[0091] The light beam image recording apparatus 10 shown in FIG. 1 whichhas the recording synchronizing signal generating unit 30 shown in FIG.2 operates and offers advantages as follows:

[0092] The rotary encoder 24 detects, as A-phase pulses Pa, theinformation of recording position in the main scanning direction X onthe PS plate 12 by the optical unit 34. Based on the A-phase pulses Pa,the PLL circuit 50 generates an original clock CKa by multiplying thefrequency of the A-phase pulses Pa.

[0093] The decimating counter 52 counts pulses of the original clockCKa, and outputs a decimating instruction Sa to decimate one pulse fromthe original clock CKa each time the count of the decimating counter 52reaches a predetermined count or decimating value Sc (Sc=72).

[0094] Based on the decimating instruction Sa, the gate circuit 54produces a clock CKb by decimating one pulse from the original clockCKa. The frequency divider 56 then frequency-divides the clock CKb at afixed frequency-dividing ratio of 8, producing and outputting a pixelclock CKi for recording an image.

[0095] Since the recording frequency of the pixel clock CKi is varied bydecimating the original clock CKa based on the decimating value Sc as apredetermined count, the image recorded on the PS plate 12 can berendered finely and accurately by determining in advance the decimatingvalue Sc depending on the positional relationship between the PS plate12 and the optical unit 34.

[0096] In FIGS. 1 and 2, the image recorded on the PS plate 12 by thelight beam L comprises a halftone dot image that is produced based onthe presence or absence of pixels, i.e., the turning on or off of thelight beam L.

[0097] The recording sheet may be a photosensitive medium such as aphotosensitive film or the like other than the PS plate 12. If theoptical unit 34 is replaced with an ink ejecting unit, then therecording sheet may be a sheet of paper.

[0098] The optical unit 34 used as the image recording means allowspixels having a diameter of 10 μm or less to be produced with the lightbeam L emitted thereby. The PS plate 12 used as the recording sheetallows the light beam image recording apparatus 10 to be constructed asa CTP (Computer To Plate) apparatus.

[0099] In the embodiment shown in FIGS. 1 and 2, the light beam imagerecording apparatus 10 is constructed as an external surface scanninglight beam image recording apparatus in which the PS plate 12 mounted onthe outer circumferential surface 16 of the drum 14 rotated by the mainscanning motor 20 is scanned in the main scanning direction X by thelight beam L emitted from the optical unit 34 to record a linear imageon the PS plate along each main scanning line, and the optical unit 34is moved in the auxiliary scanning direction Y along the shaft 18 of thedrum 14 by the auxiliary scanning motor 40 to record a two-dimensionalimage on the PS plate 12. The image recorded on the PS plate 12 canmaintain a desired level of dimensional accuracy irrespective ofvariations of the diameter of the drum 14.

[0100]FIG. 3 shows an internal scanning type light beam image recordingapparatus 90 according to another embodiment of the present invention.

[0101] As shown in FIG. 3, the internal scanning type light beam imagerecording apparatus 90 has a cylindrical drum 70 with a PS plate 12 as arecording sheet mounted on an inner circumferential surface 72 thereof,an optical unit 76 comprising a laser beam source for emitting a lightbeam L along the central axis of the drum 70, a spinner 80 disposed onthe central axis of the drum 70 and having a reflecting mirror surface78 inclined at an angle of 45° to the axis of the light beam L, a mainscanning motor 82 for rotating the spinner 80 about the central axis ofthe drum 70 at a high constant speed for scanning the PS plate 12 in amain scanning direction X to record a linear image on the PS plate alongeach main scanning line, and an auxiliary scanning system (not shown)for moving the spinner 80 in an auxiliary scanning direction Y along thecentral axis of the drum 70 to record a two-dimensional image on the PSplate 12.

[0102] The exposure control unit 28 shown in FIG. 1 is incorporated inthe internal scanning type light beam image recording apparatus 90 shownin FIG. 3 to record an image on the PS plate 12 with a desired level ofdimensional accuracy irrespective of variations of the diameter of thedrum 70.

[0103] The external surface scanning light beam image recordingapparatus shown in FIG. 1 also has a rotary encoder 24 for outputting aZ-phase pulse Pz each time the drum 14 makes one revolution, the rotaryencoder 24 functioning as a means for detecting information (origininformation) per revolution of the drum 14.

[0104] Further, the internal scanning type light beam image recordingapparatus 90 shown in FIG. 3 also has a rotary encoder 84 for outputtinga Z-phase pulse Pz each time the spinner 80 makes one revolution, therotary encoder 84 functioning as a means for detecting information(origin information) per revolution of the spinner 80.

[0105] In each of the embodiments shown in FIGS. 1, 2 and FIG. 3, whenthe decimating counter 52 detects a Z-phase pulse Pz, the decimatingcounter 52 is reset to clear the count of the original clock CKathereby, and thereafter starts to count the original clock CKa up to thepredetermined count Sd. Therefore, the first correcting value, i.e., thecount Sd, can be varied for each main scanning line thereby tofacilitate a fine correcting process.

[0106] After the decimating counter 52 is reset by a Z-phase pulse Pzand until it generates a decimating instruction Sa, the decimatingcounter 52 is set to the count Sd corresponding to the random number Segenerated by the random number generator 66 as a first count Sb forcounting the original clock CKa. After the decimating counter 52 isreset by a Z-phase pulse Pz and until it generates a second andsubsequent decimating instruction Sa, the decimating counter 52 is setto the preset count Sc as a second and subsequent count Sb for countingthe original clock CKa.

[0107] The gate circuit 54 is closed, i.e., its switch is opened, forthe period of one pulse each time a decimating instruction Sa isproduced as a counting end signal by the decimating counter 52. Sincethe interval after the decimating counter 52 is reset by a Z-phase pulsePz and until it subsequently generates a first decimating instruction Sadepends on the count Sd, which is of a value equal to or smaller thanthe count Sc and equal to the random number Se, corresponding to therandom number Se, pixel clock positions are prevented from beingcorrected in fixed positions along the main scanning direction at alltimes, so that an image produced on the PS plate 12 does not suffer aquality degradation such as a striped irregularity or a moiré pattern.

[0108] Setting the first count Sd of the original clock CKa after thedecimating counter 52 has been reset to a value between a value of 0 andthe preset count Sc (Sc=72) with the comparator 68 offers suchadvantages that the random number generator 66 is simple in structureand all corrected positions are prevented from being displaced largely,i.e., more than the count Sc.

[0109]FIG. 4 shows another recording synchronizing signal generatingunit 30A for use in the light beam image recording apparatus, which iscapable of determining the count Sc set by the CPU 64 depending on theoutside diameter of the drum 14, or the inside diameter of the drum 70,or the temperature of the light beam image recording apparatus 10, orthe temperature of the internal scanning type light beam image recordingapparatus 90, or the thickness of the PS plate 12 that may be of 0.2 mm,0.24 mm, or 0.3 mm.

[0110] As shown in FIG. 4, the recording synchronizing signal generatingunit 30A is similar to the recording synchronizing signal generatingunit 30 shown in FIG. 2, except that it has a correcting data memory 100instead of the decimating value register 62 shown in FIG. 2, andadditionally includes a temperature sensor 102 connected to the CPU 64for measuring or detecting the temperature of the light beam imagerecording apparatus, and a input means 104 such as a keyboard or thelike connected to the CPU 64 for entering the thickness of the PS plate12.

[0111] The correcting data memory 100 is supplied with Z-phase pulses Pzand A-phase pulses Pa as memory address data.

[0112] The recording synchronizing signal generating unit 30A shown inFIG. 4 has a clock adjusting means 200A for adjusting the pixel clockCKi which comprises a decimation counting means 202A and an initialdecimation random number generating means 204. The clock adjusting means200A comprises the decimating counter 52, the switch 58, the CPU 64, thecorrecting data memory 100, the temperature sensor 102, and the inputmeans 104, and the initial decimation random number generating means 204comprises the initial decimating value register 60, the random numbergenerator 66, and the comparator 68.

[0113]FIG. 5 shows a process of generating correcting data which is tobe stored in advance in the correcting data memory 100 of the recordingsynchronizing signal generating unit 30A.

[0114] In step S1 shown in FIG. 5, with the PS plate 12, whose thicknesshas been measured, being mounted on the outer circumferential surface 16of the drum 14, the temperature of the light beam image recordingapparatus 10 that is placed in a constant-temperature chamber ismeasured by the temperature sensor 102. The thickness of the PS plate 12is entered into the CPU 64 via the input means 104.

[0115] In step S2, without the decimating counter 52 operating, i.e.,with the gate circuit 54 being closed as shown in FIG. 4, the drum 14 isrotated at a constant speed, and image data for generating a referencetest chart TC shown in FIG. 6 is stored in the memory 44 (see FIG. 1)and used to record an image corresponding to the reference test chart TCentirely on the PS plate 12.

[0116] In step S3, the PS plate 12 on which the image corresponding tothe reference test chart TC (hereinafter referred to as “recorded testchart TC1”) is removed from the drum 14, and the status of the recordedtest chart TC1, i.e., the recorded test chart TC1 which is suffering adistortion, on the PS plate 12 is measured by a measuring means (notshown).

[0117] In step S4, the difference between the measured recorded testchart TC1 and the reference test chart TC is calculated, and adecimating value Sc on the drum 14, i.e., the number of pulses of theoriginal clock CKa which are to be counted before one pulse isdecimated, is calculated in order to eliminate the difference and storedin the correcting data memory 100 (see FIG. 4). The decimating value Scmay be set to a different value each time the drum 14 makes onerevolution in the main scanning direction X, and also may be set to adifferent value during one revolution of the drum 14.

[0118] If one pixel has a size of 10 μm, then when the decimating valueSc is set to Sc=72, the decimating process is performed once in 0.72 mm.

[0119] The processing in steps S1 - S4 is carried out for each ofdifferent temperature settings, each of different thicknesses of the PSplate 12, and each of different diameters of the drums 14, 70, therebyproducing correcting decimating value data. The produced correctingdecimating value data are then stored in the correcting data memory 100.

[0120] By thus determining the count Sc set in the decimating valueregister 62 by the CPU 64 depending on the outside diameter of the drum14, or the inside diameter of the drum 70, or the temperature of thelight beam image recording apparatus 10, or the temperature of theinternal scanning type light beam image recording apparatus 90, or thethickness of the PS plate 12, images to be recorded can accurately becorrected with respect to such various parameters.

[0121] The feature of the above embodiments of the present invention andthe technique disclosed in Japanese laid-open patent publication No.10-16290 will be described for comparison with reference to FIGS. 7 and13. FIG. 7 shows each of the recording synchronizing signal generatingunits according to the above embodiments in simplified block form.

[0122] In the conventional system shown in FIG. 13, the frequency of afundamental clock supplied from the rotary encoder 1 is multiplied toproduce an original clock by the PLL circuit 2. The frequency of theoriginal clock is then divided by the frequency divider 3, whosefrequency-dividing ratio is set to “7”, “8”, or “9” by the clockadjusting means 7 based on information of variations of the diameter ofthe drum, thus producing a pixel clock. However, since the correctivevalue of the frequency-dividing ratio is limited to “7”, “8”, or “9”,the ability of the conventional system to deal with small variations ofthe drum diameter is low.

[0123] In the recording synchronizing signal generating unit shown inFIG. 7, the frequency-multiplying number of the PLL circuit 50 is set tothe sufficiently large number of pulses determined from a resolution tobe corrected relative to the pixel clock CKi, e.g., about 10 μm if theresolution with which to record an image on the PS plate 12 is 2400 DPI(dots per inch), and is decimated based on the decimating value Sc andthe initial decimating value Sd by the clock adjusting means 200 (200A).

[0124] According to the foregoing process, the recording synchronizingsignal generating unit shown in FIG. 7 is capable of controlling theposition of a recording pixel in the main scanning direction X with asimple arrangement. Therefore, the recording synchronizing signalgenerating unit shown in FIG. 7 has a higher ability to deal with smallvariations of the drum diameter, or stated otherwise, is capable ofcorrecting the resolution with simple instructions.

[0125] Specifically, if the drum diameter slightly changes from a 72/72magnification (1 magnification) to a 73/72 magnification, then theconventional system needs to change the frequency-dividing ratio from acorrecting pattern of “8, 8, 8, 8, 8, 8, 8, 8, 8” (72) to “9, 8, 8, 8,8, 8, 8, 8, 8” (73). However, the recording synchronizing signalgenerating unit shown in FIG. 7 can deal with such a small variation ofthe drum diameter simply by changing the decimating value Sc stored inthe decimating value register 62 from 72 to 73.

[0126] As described above, the recording synchronizing signal generatingunit shown in FIG. 7 provides a wider correcting range with a simplearrangement than the conventional system shown in FIG. 13.

[0127] In the conventional system shown in FIG. 13, the original clockoutputted from the PLL circuit 2, whose frequency is 8 times thefrequency of the pixel clock, is used as a reference clock and usuallyfrequency-divided by 8 and partly frequency-divided by 7 or 9 by thecounter or frequency divider 3, for the correction of pixel clockpositions. Therefore, pixel clock positions are corrected in fixedpositions along the main scanning direction at all times, so that animage produced on the photosensitive medium such as the PS plate tendsto suffer a quality degradation such as a striped irregularity or amoiré pattern. In the recording synchronizing signal generating unitshown in FIG. 7, however, since the initial decimating value Sd is arandom number, a quality degradation such as a striped irregularity or amoiré pattern due to optical conflict with a halftone screen isprevented as much as possible.

[0128] The principles of the light beam image recording apparatus 10shown in FIG. 1 are also applicable to an external scanning type ink jetimage recording apparatus 110 shown in FIG. 8 by replacing the exposurecontrol unit 28, the LD driver 46, and the optical unit 34 of the lightbeam image recording apparatus 10 shown in FIG. 1 respectively with anejection control unit 128, an ink jet driver 146, and an ink jet head134 shown in FIG. 8. The external scanning type ink jet image recordingapparatus 110 can directly be used as a printing press such as an offsetprinting press.

[0129] If the external scanning type ink jet image recording apparatus110 is used as a printing press, then a recording sheet 112 such as analuminum plate whose surface has been treated to attain a hydrophilicnature is mounted on the drum 14 that is used as a plate cylinder. Then,the ink jet head 134 ejects a lipophilic ink I based on the image dataDi to the recording sheet 112 on the drum 14 as it rotates in the mainscanning direction X to form a linear image along each main scanningline. At the same time, the ink jet head 134 is moved in the auxiliaryscanning direction Y to record a two-dimensional image, which iscomposed of areas where the lipophilic ink I is applied and areas wherethe lipophilic ink I is not applied, on the recording sheet 112.

[0130] In this manner, a halftone dot image is formed of the appliedareas of the lipophilic ink I on the hydrophilic recording sheet 112.

[0131] For printing the image, damping water is applied to the recordingsheet 112 by a water roller (not shown), and then a printing ink isapplied to the recording sheet 112 by a printing ink roller (not shown).The printing ink is attached to only the halftone dot image of theapplied areas of the lipophilic ink I. The applied printing ink is thentransferred to a printing sheet of paper, thus producing a printedmaterial with the halftone dot image.

[0132] The external scanning type ink jet image recording apparatus 110is advantageous in that it requires no developing and fixing process fordeveloping and fixing the recorded image.

[0133] As described above, the present invention is applicable tovarious image recording apparatus which need to make corrections withrespect to the drum system such as light beam image recording apparatusand ink jet image recording apparatus.

[0134] As described above, since the recording frequency is varied bydecimating pulses of the original clock at locations where need to becorrected, such corrections can be made stably with a simple arrangementagainst a distortion of an image, such as an expansion or a contraction,due to an error of a mechanical system for holding a recording sheet,e.g., a drum or the like.

[0135] In this fashion, a more accurate image can be recorded on andreproduced from a recording sheet.

[0136] Inasmuch as the first decimated position in the main scanningdirection is determined by a random number, the image recordingapparatus does not produce a quality degradation such as a stripedirregularity or a moiré pattern in the image recorded on the recordingsheet.

[0137]FIG. 9 shows in block form still another recording synchronizingsignal generating unit 30B, which functions as a pixel clock generatingmeans, for use in the light beam image recording apparatus.

[0138] Those parts of the recording synchronizing signal generating unit30B which are identical to those shown in FIGS. 1 through 8 are denotedby identical reference characters, and will not be described in detailbelow.

[0139] As shown in FIG. 9, the recording synchronizing signal generatingunit 30B has a PLL circuit 50 functioning as an original clockgenerating means for generating an original clock CKa that comprisesmultiplied pulses (multiplied by 10) in synchronism with the A-phasepulses Pa.

[0140] The original clock CKa is supplied to a count input terminal of adecimating counter 52, which functions as a decimation counting meansfor counting pulses of the original clock CKa and outputting adecimating instruction Sa to decimate a pulse from the original clockCKa each time a preset count Sb is reached. The original clock CKa isalso supplied to an input terminal 54 b of a gate circuit 54, whichfunctions as a decimating means for decimating a pulse from the originalclock CKa and outputting a clock CKb from which the pulse has beendecimated according to the decimating instruction Sa.

[0141] The clock CKb is supplied from an output terminal 54 a of thegate circuit 54 to a frequency divider 56, which functions as afrequency-dividing means whose frequency-dividing ratio is fixed to “8”,for example. The frequency divider 56 frequency-divides the clock CKb by8, and outputs a pixel clock CKi for recording an image, i.e., outputsone pulse of the pixel clock CKi each time 8 pulses of the clock CKb aresupplied to the frequency divider 56.

[0142] The decimating counter 52 comprises a preset down counter, andhas a reset input terminal supplied with a Z-phase pulse Pz each timethe drum 14 makes one revolution.

[0143] When a Z-phase pulse Pz is supplied to the decimating counter 52,the decimating counter 52 resets, i.e., clears, the count of theoriginal clock Cka, and starts counting pulses of the original clock Ckaagain.

[0144] The decimating counter 52 has a preset input terminal 52 a whichis supplied with a count Sb calculated by a decimating value calculatingmeans 308.

[0145] When pulses of the original clock CKa are counted to the count Sbby the decimating counter 52, the decimating counter 52 outputs acounting end signal as the decimating instruction Sa to a controlterminal 54 d of the gate circuit 54, shifting the output terminal 54 ato an unconnected terminal 54 c for a period of time in which one pulseof the original clock CKa is decimated.

[0146] When the counting end signal as the decimating instruction Sa issupplied to the decimating value calculating means 308, the decimatingvalue calculating means 308 supplies a new count Sb to the decimatingcounter 52.

[0147] A mechanical component data memory 100 as a storage means storesmechanical component correcting data Dm, and a proportional componentdata register 306 stores proportional component correcting data Dp. Themechanical component correcting data Dm outputted from the mechanicalcomponent data memory 100 and the proportional component correcting dataDp outputted from the proportional component data register 306 arecombined into a combined correcting value (multiplied value) Dp·Dm by acombining means 309. The combined correcting value Dp·Dm is supplied tothe decimating value calculating means 308. The decimating valuecalculating means 308 refers to the combined-correcting value Dp·Dm anda reference value Ds, e.g., Ds=72, and calculates the count Sb as adecimating value.

[0148] By standardizing central values of the mechanical componentcorrecting data Dm and the proportional component correcting data Dp toa value of “1”, the count Sb can be calculated as Sb=[Ds×Dp·Dm] (thespecial function [x] which is a mathematical symbol means the integralpart of x).

[0149] If one pixel has a size of 10 μm, then when the decimating valueSc is set to Sc=72, the decimating process is performed once in 0.72 mm.

[0150] The mechanical component data memory 100 stores two-dimensionalmechanical component correcting data, which have been standardized, inthe main and auxiliary scanning directions X, Y, including parts andassembling tolerances based on the relative positional relationshipbetween the outer circumferential surface 16 of the drum 14 and theoptical unit 34. The two-dimensional mechanical component correctingdata are supplied via the input means 104 and the CPU 64 to themechanical component data memory 100.

[0151] A-phase counter 302, functioning as a counting means, countsA-phase pulses Pa outputted from the rotary encoder 24, and a Z-phasecounter 304, functioning as a counting means, counts Z-phase pulses Pzfrom the rotary encoder 24. The mechanical component data memory 100outputs mechanical component correcting data Dm at an address indicatedby the count outputs from the A-phase counter 302 and the Z-phasecounter 304.

[0152] The proportional component correcting data Dp stored in theproportional component data register 306 are set by the CPU 64 based onthe resolution (dpi: dots per inch) of pixels recorded on the PS plate12 by the light beam L, the thickness of the PS plate 12 that may be of0.2 mm, 0.24 mm, or 0.3 mm, and the temperature or humidity in the lightbeam image recording apparatus 10, and are represented byproportionality constants.

[0153] For recording an image on the PS plate 12, the CPU 64 determinesa proportionality constant based on the temperature information in theapparatus 10 from the temperature sensor 102 and thickness andresolution information of the PS plate 12 which is entered by theoperator via the input means 104, and stores the determinedproportionality constant as proportional component correcting data Dp inthe proportional component data register 306. The resolution informationmay be entered from a RIP (Raster Image Processor), not shown. The CPU64 may determine a proportionality constant based on at least one of thetemperature information, the thickness information, and the resolutioninformation, and store the determined proportionality constant asproportional component correcting data Dp in the proportional componentdata register 306.

[0154]FIG. 10 shows a process of generating mechanical componentcorrecting data Dm which is to be stored in advance in the mechanicalcomponent data memory 100.

[0155] In step S11 shown in FIG. 10, with the PS plate 12 whosethickness has been measured being mounted on the outer circumferentialsurface 16 of the drum 14, the temperature of the light beam imagerecording apparatus 10 that is placed in a constant-temperature chamberis set to a standard temperature of 20° C. and measured by thetemperature sensor 102. The thickness of the PS plate 12 is entered intothe CPU 64 via the input means 104.

[0156] In step S12, without the decimating counter 52 operating, i.e.,with the gate circuit 54 being closed as shown in FIG. 9, the drum 14 isrotated at a constant speed, and image data for generating a referencetest chart TC shown in FIG. 6 is stored in the memory 44 (see FIG. 1)and used to record an image corresponding to the reference test chart TCentirely on the PS plate 12.

[0157] In step S13, the PS plate 12 on which the image corresponding tothe reference test chart TC (hereinafter referred to as “recorded testchart TC1”) is removed from the drum 14, and the status of the recordedtest chart TC1 on the PS plate 12, i.e., the recorded test chart TC1which is suffering a distortion, is measured by a measuring means (notshown).

[0158] In step S14, the difference between the measured recorded testchart TC1 and the reference test chart TC is calculated, and aproportionality constant to eliminate the difference is calculated foreach grid point and stored as mechanical component correcting data Dm inthe mechanical component data memory 100.

[0159] To determine proportional component correcting data Dp to bestored in the proportional component data register 306, the processingin steps S11 - S14 is carried out for each of different temperaturesettings, each of different thicknesses of the PS plate 12, and each ofdifferent diameters of the drum 14, thereby determining a formula forcalculating proportional component correcting data Dp representingproportionality constants for the mechanical component correcting dataDm. The determined formula is stored in a rewritable read-only memory(ROM) such as a flash memory in the CPU 64.

[0160] The light beam image recording apparatus 10 shown in FIG. 1 whichhas the recording synchronizing signal generating unit 30B shown in FIG.9 operates and offers advantages as follows:

[0161] The thickness and recording resolution information of the PSplate 12 are entered via the input means 104 into the CPU 64, and thetemperature information is entered into the CPU 64. The CPU 64calculates proportional component correcting data Dp from these suppliedinformation, and stores the calculated proportional component correctingdata Dp in the proportional component data register 306. Then, the mainscanning motor 20 rotates the drum 14 at a constant speed. At this time,one of the input terminals of the combining means 309 is supplied withthe calculated proportional component correcting data Dp stored in theproportional component data register 306.

[0162] While the drum 14 is rotating at a constant speed, the rotaryencoder 24 detects, as A-phase pulses Pa, the information of recordingposition in the main scanning direction X on the PS plate 12 by theoptical unit 34. Based on the A-phase pulses Pa, the PLL circuit 50generates an original clock CKa by multiplying the frequency of theA-phase pulses Pa.

[0163] The mechanical component correcting data Dm is read from themechanical component data memory 100 at an address based on the outputcount from the A-phase counter 302 and the output count from the Z-phasecounter 304, and supplied to the other input terminal of the combiningmeans 309. The combining means 309 combines, i.e., multiplies themechanical component correcting data Dm and the proportional componentcorrecting data Dp, and supplies the product, i.e., a combinedcoefficient (beam position aligning coefficient) Dp·Dm to the decimatingvalue calculating means 308.

[0164] The decimating value calculating means 308 determining adecimating value Sb at the time from the combined coefficient Dp·Dm andthe reference value Ds, and sets the decimating value Sb as a presetvalue in the decimating counter 52.

[0165] The original clock CKa generated by the PLL circuit 50 is countedby the decimating counter 52, which outputs a decimating instruction Safor decimating one pulse from the original clock CKa each time the countof the decimated counter 52 reaches the preset count or decimating valueSb.

[0166] Based on the decimating instruction Sa, the gate circuit 54produces a clock CKb by decimating one pulse from the original clockCKa. The frequency divider 56 then frequency-divides the clock CKb at afixed frequency-dividing ratio of 8, producing and outputting a pixelclock CKi for recording an image.

[0167] In this embodiment, when no pulse is decimated, the pixel clockCKi has a time interval corresponding to 8 pulses of the original clockCKa outputted from the PLL circuit 50. When one pulse is decimated, thepixel clock CKi has a time interval corresponding to 9 pulses of theoriginal clock CKa. The increased time interval is equivalent to a pixelsize that is 9/8 times the original pixel size, elongating the imagelocally by the increased time interval.

[0168] The decimating counter 52 is reset each time a Z-phase pulse Pzis supplied, i.e., each time the drum 14 makes one revolution.

[0169] Since the recording frequency of the pixel clock CKi is varied bydecimating the original clock CKa based on the decimating value Sb as apredetermined count, the image recorded on the PS plate 12 can berendered finely and accurately by determining in advance the decimatingvalue Sb depending on the positional relationship between the PS plate12 and the optical unit 34. The image recorded on the PS plate 12 by thelight beam L in FIGS. 1 and 9 comprises a halftone dot image that isproduced based on the presence or absence of pixels, i.e., the turningon or off of the light beam L.

[0170] The recording sheet may be a photosensitive medium such as aphotosensitive film or the like other than the PS plate 12. If theoptical unit 34 is replaced with an ink ejecting unit, then therecording sheet may be a sheet of paper.

[0171] The optical unit 34 used as the image recording means allowspixels having a diameter of 10 μm or less to be produced with the lightbeam L emitted thereby. The PS plate 12 used as the recording sheetallows the light beam image recording apparatus 10 to be constructed asa CTP (Computer To Plate) apparatus.

[0172] In the embodiment shown in FIGS. 1 and 9, the light beam imagerecording apparatus 10 is constructed as an external surface scanninglight beam image recording apparatus in which the PS plate 12 mounted onthe outer circumferential surface 16 of the drum 14 rotated by the mainscanning motor 20 is scanned in the main scanning direction X by thelight beam L emitted from the optical unit 34 to record a linear imageon the PS plate along each main scanning line, and the optical unit 34is moved in the auxiliary scanning direction Y along the shaft 18 of thedrum 14 by the auxiliary scanning motor 40 to record a two-dimensionalimage on the PS plate 12. The image recorded on the PS plate 12 canmaintain a desired level of dimensional accuracy irrespective ofvariations of the diameter of the drum 14.

[0173] The recording synchronizing signal generating unit shown in FIGS.1 and 9 can be incorporated in the internal scanning type light beamimage recording apparatus 90 shown in FIG. 3.

[0174] In the embodiment shown in FIGS. 1 and 9, when the decimatingcounter 52 detects a Z-phase pulse Pz outputted from the rotary encoder24, 84 which detects each revolution of the drum 14, 70, the decimatingcounter 52 is reset to clear its count and then starts counting theoriginal clock CKa up to the predetermined count Sd. The firstcorrecting value, i.e., the count Sb, can be varied for each mainscanning line thereby to facilitate a fine correcting process.

[0175]FIG. 11 shows yet another recording synchronizing signalgenerating unit 30C which is capable of varying a preset count Sd whichis the first decimating value for each main scanning line.

[0176] In FIG. 11, the preset count Sb which is the first decimatingvalue calculated by the decimating value calculating means 308 is storedin the decimating value register 62.

[0177] The recording synchronizing signal generating unit 30C is similarto the recording synchronizing signal generating unit 30B shown in FIG.9 except that a switch 58 which comprises one-circuit, two-contactswitch is inserted between the decimating value register 62 and thedecimating counter 52. The switch 58 has control terminals 58 d, 58 e, acommon terminal 58 a, and fixed terminals 58 b, 58 c.

[0178] A count Sc stored in the decimating value register 62 is suppliedto the fixed terminal 58 b of the switch 58, and an initial count Sdstored as a first count in an initial decimating value register 60,which functions as an initial count setting means, of an initialdecimation random number setting means 204, is supplied to the fixedterminal 58 c of the switch 58.

[0179] When a Z-phase pulse Pz is supplied to the control terminal 58 dof the switch 58, the common terminal 58 a is temporarily connected tothe fixed terminal 58 c, supplying the initial count Sd as a count Sb(Sb=Sd) to a preset input terminal of the decimating counter 52.

[0180] When pulses of the original clock CKa are counted to the firstcount Sd by the decimating counter 52, the decimating counter 52 outputsa counting end signal as the decimating instruction Sa to the controlterminal 54 d of the gate circuit 54, shifting the output terminal 54 ato the unconnected terminal 54 c for a period of time in which one pulseof the original clock CKa is decimated.

[0181] The counting end signal as the decimating instruction Sa is alsosupplied to the control terminal 58 e of the switch 58, shifting thecommon terminal 58 a from the fixed terminal 58 c to the fixed terminal58 b to allow a second and subsequent count Sc which has been set in thedecimating value register 62 to be set as the preset count Sb in thesetting terminal of the decimating counter 52 (Sb=Sc).

[0182] A random number Se which is equal to or smaller than the secondand subsequent count Sc is set as a first count Sd in each main scanningline in the initial decimating value register 60.

[0183] If a random number Se generated by the random number generator 66that is initiated by a Z-phase pulse Pz or a disagreement output Sf fromthe comparator 68 is equal to or smaller than the count Sc set in thedecimating value register 62 (Se≦Sc), then the random number Se is setas the first count Sd in the initial decimating value register 60. Ifthe random number Se is in excess of the count Sc set in the decimatingvalue register 62, then a random number Se is generated again by therandom number generator 66 based on the disagreement output Sf from thecomparator 68 until the random number Se becomes equal to or smallerthan the count Sc.

[0184] After the decimating counter 52 is reset by a Z-phase pulse Pzand until it generates a decimating instruction Sa, the decimatingcounter 52 is set to the count Sd corresponding to the random number Segenerated by the random number generator 66 as a first count Sb forcounting the original clock CKa. After the decimating counter 52 isreset by a Z-phase pulse Pz and until it generates a second andsubsequent decimating instruction Sa, the decimating counter 52 is setto the preset count Sc as a second and subsequent count Sb for countingthe original clock CKa.

[0185] The gate circuit 54 is closed, i.e., its switch is opened, forthe period of one pulse each time a decimating instruction Sa isproduced as a counting end signal by the decimating counter 52. Sincethe interval after the decimating counter 52 is reset by a Z-phase pulsePz and until it subsequently generates a first decimating instruction Sadepends on the count Sd, which is of a value equal to or smaller thanthe count Sc and equal to the random number Se, corresponding to therandom number Se, pixel clock positions are prevented from beingcorrected in fixed positions along the main scanning direction at alltimes, so that an image produced on the PS plate 12 does not suffer aquality degradation such as a striped irregularity or a moiré pattern.

[0186] Setting the first count Sd of the original clock CKa after thedecimating counter 52 has been reset to a value between a value of 0 andthe preset count Sc with the comparator 68 offers such advantages thatthe random number generator 66 is simple in structure and all correctedpositions are prevented from being displaced largely, i.e., more thanthe count Sc.

[0187]FIG. 12 shows yet still another recording synchronizing signalgenerating unit 30D. The recording synchronizing signal generating unit30D differs from the recording synchronizing signal generating unit 30Cshown in FIG. 11 in that the mechanical component data memory 100 isdivided into a main scanning component memory 100 a and an auxiliaryscanning component memory 100 b, and the combining means 309 whichcomprises a multiplier is divided into two combining means 309 a, 309 awhich comprise respective multipliers.

[0188] The main scanning component memory 100 a stores main scanningcomponent correcting data Dmm for correcting a graphical accuracydistortion in the circumferential direction of the drum 14, and theauxiliary scanning component memory 100 b stores auxiliary scanningcomponent correcting data Dmo for correcting a graphical accuracydistortion in the axial direction of the drum 14. The main scanningcomponent correcting data Dmm outputted from the main scanning componentmemory 100 a, the auxiliary scanning component correcting data Dmooutputted from the auxiliary scanning component memory 100 b, and theproportional component correcting data Dp outputted from theproportional component data register 306 are combined into a combinedcorrecting value (multiplied value) Dp·Dmm·Dmo by the combining means309 a, 309 b. The combined correcting value Dp·Dmm·Dmo is supplied tothe decimating value calculating means 308.

[0189] The decimating value calculating means 308 refers to the combinedcorrecting value Dp·Dmm·Dmo and the reference value Ds, e.g., Ds=72, andcalculates the count Sb as a decimating value. Specifically, the countSb can be calculated as Sb=[Ds×Dp·Dmm·Dmo] (the special function [x]which is a mathematical symbol means the integral part of x).

[0190] In the embodiment shown in FIG. 12, the correcting data areclassified into three components independent of each other.

[0191] Specifically, the first component is the proportional componentcorrecting data Dp for correcting pixel intervals depending on thetemperature and humidity of the apparatus, the thickness of the PS plate12, and the recording resolution which are independent of the recordingpositions of the image data. The proportional component correcting dataDp is set and stored in the proportional component data register 306from the CPU 64 each time an image is to be recorded. That is, theproportional component correcting data Dp is not kept continuously, butis calculated and stored in the proportional component data register 306by the CPU 64 each time exposure conditions are determined when an imageis to be recorded by the light beam.

[0192] The second component is the main scanning component correctingdata (coefficient) Dmm for compensating for variations of the diameters,eccentricities, and shaft oscillations of the drums 14, 70, and diskeccentricities of the rotary encoders 24, 84, which are reproduced inthe circumferential direction, i.e., the main scanning direction X ofthe drums 14, 70. The main scanning component correcting data Dmm isproduced and stored as follows: When the light beam image recordingapparatus 10, 90 are assembled, distortions in the main scanningdirection X of an image recorded on the PS plate 12 by the light beam Lwhich is emitted from the optical unit 34 and the spinner 80 based onthe standard pixel clock from which no pulses are decimated aremeasured, and the measured distortions are stored as the main scanningcomponent correcting data Dmm inherent in the apparatus in the mainscanning component memory 100 a. With the main scanning componentcorrecting data Dmm thus determined, the storage capacity of the mainscanning component memory 100 a may be as large as one or several morelines in the main scanning direction X, and may be much smaller than ifthe mechanical component correcting data are stored in a two-dimensionalfashion in the mechanical component data memory 100.

[0193] The third component is the auxiliary scanning componentcorrecting data (coefficient) Dmo for compensating for variations in theauxiliary scanning direction Y of the diameters of the drums 14, 70,which are reproduced in the direction, i.e., the auxiliary scanningdirection Y of the drums 14, 70, that is substantially perpendicular tothe circumferential direction of the drums 14, 70. The auxiliaryscanning component correcting data Dmo is produced and stored asfollows: When the light beam image recording apparatus 10, 90 areassembled, distortions in the auxiliary scanning direction Y of an imagerecorded on the PS plate 12 based on the standard pixel clock from whichno pulses are decimated are measured, and the measured distortions arestored as the auxiliary scanning component correcting data Dmo inherentin the apparatus in the auxiliary scanning component memory 100 b. Withthe auxiliary scanning component correcting data Dmo thus determined,the storage capacity of the auxiliary scanning component memory 100 bmay be as large as one line in the auxiliary scanning direction Y, andmay be much smaller than if the mechanical component correcting data arestored in a two-dimensional fashion in the mechanical component datamemory 100.

[0194] When recording conditions such as exposure conditions aredetermined at the time of actually recording an image on the PS plate12, the proportional component correcting data Dp is determined. When animage is to be recorded, the main scanning component correcting data Dmmand the auxiliary scanning component correcting data Dmo are read fromthe main scanning component memory 100 a and the auxiliary scanningcomponent memory 100 b according to the output counts from the A-phasecounter 302 and the Z-phase counter 304. The combined correcting value(multiplied value) Dp·Dmm·Dmo (the data Dp, Dmm, Dmo may be multipliedin any desired order) is supplied to the decimating value calculatingmeans 308, which calculates the decimating value Sc from the combinedcorrecting value Dp·Dmm·Dmo and the standard value Ds.

[0195] In the above embodiment, error factors such as variations of thediameters and eccentricities of the drums 14, 70, which are caused whenthe apparatus is assembled, and error factors such as the temperature,humidity, and recording resolution, which are caused each time an imageis recorded by the light beam L, can separately be kept or set, forthereby reducing the burden of the calculating process to be carried outbefore an image is recorded.

[0196] Even if the diameters of the drums 14, 70 suffer variations, thepixel clock CKi for maintaining a desired level of graphical accuracyfor recording on the PS plate 12 can be generated highly accurately witha simple arrangement.

[0197] In the above embodiment, as shown in FIGS. 11 and 12, the initialdecimation random number generating means 204 is provided to determinethe first decimating value, i.e., decimating position, in each mainscanning cycle with a random number thereby to decimate a pulse at adifferent position in each line. Therefore, the recorded image isprevented from suffering a quality degradation such as a stripedirregularity or a moiré pattern due to optical conflict with a halftonescreen.

[0198] The principles of the light beam image recording apparatus 10shown in FIG. 1 which incorporates either one of the recordingsynchronizing signal generating units 30B, 30C, 30D shown in FIGS. 9,11, and 12 are also applicable to the external scanning type ink jetimage recording apparatus 110 shown in FIG. 8 by replacing the exposurecontrol unit 28, the LD driver 46, and the optical unit 34 respectivelywith the ejection control unit 128, the ink jet driver 146, and the inkjet head 134 shown in FIG. 8. The external scanning type ink jet imagerecording apparatus 110 can directly be used as a printing press such asan offset printing press.

[0199] According to the above embodiment of the present invention, datafor correcting the level of graphical accuracy are generated and heldefficiently with a resource saver based on mechanical componentcorrecting data and proportional component correcting data against adistortion of an image, such as an expansion or a contraction, due to anerror of a mechanical system for holding a recording sheet.

[0200] Specifically, when the image recording means scans the recordingsheet mounted on the drum, which is a mechanical component, that isrotating at a constant speed, in the main scanning direction to recordan image per each pixel clock pulse, and is moved in the auxiliaryscanning direction to record a two-dimensional image on the recordingsheet, the time to generating a pixel clock for energizing the imagerecording means is controlled based on mechanical component correctingdata based on the relative positional relationship between themechanical component and the image recording means and proportionalcomponent correcting data corresponding to recording conditions such asthe temperature and humidity at the time of recording the image. Themechanical component correcting data include main and auxiliary scanningdirection components each kept for one line, and the proportionalcomponent correcting data is not kept but recalculated each timerecording conditions are determined. Therefore, the amount of correctingdata that is generated and held is minimized.

[0201] According to the present invention, consequently, the pixel clockfor accurately plotting the image recorded on the recording sheet by theimage recording means can be generated by a simple arrangement.

[0202] Since the first correcting process in the main scanning directionis carried out based on a random number, the image is prevented fromsuffering a quality degradation such as a striped irregularity or amoiré pattern.

[0203] Although certain preferred embodiments of the present inventionhave been shown and described in detail, it should be understood thatvarious changes and modifications may be made therein without departingfrom the scope of the appended claims.

What is claimed is:
 1. An image recording apparatus comprising: imagerecording means for scanning a recording sheet in a main scanningdirection to record an image on the recording sheet, said imagerecording means being movable in an auxiliary scanning directionsubstantially perpendicular to the main scanning direction to record atwo-dimensional image on the recording sheet; means for detectingrecording position information in said main scanning direction; originalclock generating means for generating an original clock based on saidrecording position information in said main scanning direction;decimation counting means for counting pulses of said original clock andoutputting a decimating instruction to decimate a pulse from saidoriginal clock each time a preset count is reached; decimating means fordecimating a pulse from said original clock based on said decimatinginstruction; and frequency-dividing means for frequency-dividing adecimated clock at a fixed frequency-dividing ratio and outputting thefrequency-divided clock as a pixel clock for recording the image.
 2. Animage recording apparatus according to claim 1, wherein said imagerecording means comprises an optical system for emitting a light beam tobe applied to said recording sheet.
 3. An image recording apparatusaccording to claim 1, wherein said image recording means comprises anink jet head for applying ink to said recording sheet, furthercomprising: a rotatable drum with said recording sheet mounted on anouter circumferential surface thereof; means for controlling said inkjet head to apply the ink to scan the recording sheet on said rotatabledrum in the main scanning direction to record the image on the recordingsheet; and means for moving said ink jet head in said auxiliary scanningdirection along an axis of said rotatable drum to record thetwo-dimensional image on the recording sheet.
 4. An image recordingapparatus according to claim 2, further comprising: a rotatable drumwith said recording sheet mounted on an outer circumferential surfacethereof; means for controlling said optical system to apply the lightbeam to scan the recording sheet on said rotatable drum in the mainscanning direction to record the image on the recording sheet; and meansfor moving said optical system in said auxiliary scanning directionalong an axis of said rotatable drum to record the two-dimensional imageon the recording sheet.
 5. An image recording apparatus according toclaim 2, further comprising: a drum with said recording sheet mounted onan inner circumferential surface thereof; means for rotating saidoptical system about an axis of said drum to cause the light beamemitted from said optical system to scan the recording sheet on saidrotatable drum in the main scanning direction to record the image on therecording sheet; and means for moving said optical system in saidauxiliary scanning direction along the axis of said drum to record thetwo-dimensional image on the recording sheet.
 6. An image recordingapparatus according to claim 4, further comprising: means for detectinginformation per revolution of said drum; said decimation counting meanscomprising means for resetting the count of said original clock andthereafter starting to count said original clock to said preset countwhen said information per revolution of said drum is detected.
 7. Animage recording apparatus according to claim 5, further comprising:means for detecting information per revolution of said optical system;said decimation counting means comprising means for resetting the countof said original clock and thereafter starting to count said originalclock to said preset count when said information per revolution of saidoptical system is detected.
 8. An image recording apparatus according toclaim 6, further comprising: random number generating means forgenerating a random number; said decimation counting means comprisingmeans for setting a first preset count of said original clock after thecount is reset to a value corresponding to the random number generatedby said random number generating means, and outputting a decimatinginstruction to set a second and subsequent preset count of said originalcount to said preset count.
 9. An image recording apparatus according toclaim 7, further comprising: random number generating means forgenerating a random number; said decimation counting means comprisingmeans for setting a first preset count of said original clock after thecount is reset to a value corresponding to the random number generatedby said random number generating means, and outputting a decimatinginstruction to set a second and subsequent preset count of said originalcount to said preset count.
 10. An image recording apparatus accordingto claim 8, wherein said first preset count of said original clock afterthe count is reset is set to said random number between a value of 0 andsaid preset value.
 11. An image recording apparatus according to claim4, wherein said preset count is determined depending on either one of adiameter of said drum, a temperature of the image recording apparatus,or a thickness of said recording sheet.
 12. A method of generating apixel clock to correct a graphical accuracy distortion of an imagerecorded on a recording sheet in an image recording apparatus which hasimage recording means for scanning a recording sheet mounted on amechanical component in a main scanning direction to record an image onthe recording sheet per pulse of the pixel clock, said image recordingmeans being movable in an auxiliary scanning direction substantiallyperpendicular to the main scanning direction to record a two-dimensionalimage on the recording sheet, said method comprising the steps of:generating (S1 - S4) mechanical component correcting data based on arelative positional relationship between said mechanical component andsaid image recording means; and controlling a time to generate the pixelclock to energize said image recording means based on said mechanicalcomponent correcting data and proportional component correcting datacorresponding to recording conditions for said image recording means torecord the image on said recording sheet, when the image is recorded onsaid recording sheet by said image recording means.
 13. A methodaccording to claim 12, wherein said image recording means comprises anoptical system for emitting a light beam to be applied to said recordingsheet.
 14. A method according to claim 12, wherein said image recordingmeans comprises an ink jet head for applying an ink to said recordingsheet, said image recording apparatus further comprising: a rotatabledrum with said recording sheet mounted on an outer circumferentialsurface thereof; means for controlling said ink jet head to apply theink to scan the recording sheet on said rotatable drum in the mainscanning direction to record the image on the recording sheet; and meansfor moving said ink jet head in said auxiliary scanning direction alongan axis of said rotatable drum to record the two-dimensional image onthe recording sheet.
 15. A method according to claim 13, wherein saidmechanical component comprises a rotatable drum with said recordingsheet mounted on an outer circumferential surface thereof, said imagerecording apparatus further comprising: means for controlling saidoptical system to apply the light beam to scan the recording sheet onsaid rotatable drum in the main scanning direction to record the imageon the recording sheet; and means for moving said optical system in saidauxiliary scanning direction along an axis of said rotatable drum torecord the two-dimensional image on the recording sheet.
 16. A methodaccording to claim 13, wherein said mechanical component comprises arotatable drum with said recording sheet mounted on an innercircumferential surface thereof, said image recording apparatus furthercomprising: means for rotating said optical system about an axis of saiddrum to cause the light beam emitted from said optical system to scanthe recording sheet on said rotatable drum in the main scanningdirection to record the image on the recording sheet; and means formoving said optical system in said auxiliary scanning direction alongthe axis of said drum to record the two-dimensional image on therecording sheet.
 17. A method according to claim 15, wherein saidmechanical component correcting data are generated as main scanningcomponent correcting data for correcting a graphical accuracy distortionin a circumferential direction of said drum, and auxiliary scanningcomponent correcting data for correcting a graphical accuracy distortionin an axial direction of said drum.
 18. A method according to claim 16,wherein said mechanical component correcting data are generated as mainscanning component correcting data for correcting a graphical accuracydistortion in a circumferential direction of said drum, and auxiliaryscanning component correcting data for correcting a graphical accuracydistortion in an axial direction of said drum.
 19. An image recordingapparatus comprising: image recording means for scanning a recordingsheet mounted on a mechanical component in a main scanning direction torecord an image on the recording sheet per pulse of a pixel clock, saidimage recording means being movable in an auxiliary scanning directionsubstantially perpendicular to the main scanning direction to record atwo-dimensional image on the recording sheet; means for detectingrecording position information in said main scanning direction; originalclock generating means for generating an original clock based on saidrecording position information in said main scanning direction;decimation counting means for counting pulses of said original clock andoutputting a decimating instruction to decimate a pulse from saidoriginal clock each time a preset count is reached; decimating means fordecimating a pulse from said original clock based on said decimatinginstruction; frequency-dividing means for frequency-dividing a decimatedclock at a fixed frequency-dividing ratio and outputting thefrequency-divided clock as a pixel clock for recording the image;storage means for storing mechanical component correcting data based ona relative positional relationship between said mechanical component andsaid image recording means; and decimating value calculating means forcalculating said preset count from said mechanical component correctingdata stored in said storage means and proportional component correctingdata corresponding to recording conditions for said image recordingmeans to record the image on said recording sheet, and setting thecalculated preset count in said decimation counting means.
 20. An imagerecording apparatus according to claim 19, further comprising: means fordetecting information per revolution of said drum; and random numbergenerating means for generating a random number; said decimationcounting means comprising means for, when the count of said originalclock is reset and thereafter said original clock starts to be countedto said preset count when said information per revolution of said drumis detected, setting a first preset count of said original clock afterthe count is reset to a value corresponding to the random numbergenerated by said random number generating means, and outputting adecimating instruction to set a second and subsequent preset count ofsaid original count to said preset count.